The best guide I have to what future life-forms will be like compared to me, if allowed to evolve naturally, is to consider what I am like compared to a fruit fly, or to bacteria.

This is true but not relevant. It suggests that future life forms will be much more complex, intelligent, powerful in changing the physical universe on many scales, good at out-competing (or predating on) other species to the point of driving them to extinction. You might also add differences between yourself and flies (and bacteria) like "future life forms will be a lot bigger and longer-lived", or you might consider those incidental because you don't value them as much.

But none of that implies anything about the future life-forms' values, except that they will be selfish to the exclusion of other species which are not useful or beautiful to them, so that old-style humans will be endangered. It doesn't imply anything that would cause me to expect to value these future species more than I value today's nonhuman species, let alone today's humans.

If you object that of course I will value myself more highly than I value a bacterium, and that I fail to adequately respect bacterial values, I can compare an algae to an oak tree. The algae is more closely-related to me; yet I still consider the oak tree a grander life form, and would rather see a world with algae and oak trees than one with only algae.

So you value other life-forms proportionally to how similar they are to you, and an important component of that is some measure of compexity, plus your sense of aesthetics (grandeur). You don't value evolutionary relatedness highly. I feel the same way (I value a cat much more than a bat (edit: or rat)), but so what? I don't see how this logically implies that new lifeforms that will exist in the future, and their new values, are more likely than not to be valued by us (if we live long enough to see them).

It's also possible that life does not naturally progress indefinitely

Life may keep changing indefinitely, barring a total extinction. But that constant change isn't "progress" by any fixed set of values because evolution has no long term goal.

Apart from the nonexistence of humans, who are unique in their intelligence/self-consciousness/tool-use/etc., life on Earth was apparently just as diverse and grand and beautiful hundreds of millions of years ago as it is today. There's been a lot of change, but no progress in terms of complexity before the very quick evolution of humans. If I were to choose between this world, and a world with humans but otherwise the species of 10, 100, or 300 millions of years ago, I don't feel that today's bio-sphere is somehow better. So I don't feel a hypothetical biosphere of 300 million years in the future would likely be better than today's on my existing values. And I don't understand why you do.

If you like, I can say that I value complexity

Do you really value complexity for its own sake? Or do you value it for the sake of the outcomes (such as intelligence) which it helps produce?

If you are offered prosthetic arms that look and feel just like human ones but work much better in many respects, you might accept them or not, but I doubt the ground for your objection would be that the biological version is much more complex.

build an FAI that maximizes some complexity measure.

Could you explain what kind of complexity measure you have in mind?. For instance, info-theoretical complexity (~ entropy) is maximized by a black hole, and is greatly increased just by a good random number generator. Surely that's not what you mean.